This intensive methods of raising animals for food (meat, eggs & milk) under modern agricultural conditions make it imperative to immunize actively against a wide range of diseases.
Avian and mammalian species are susceptible to many disease causing organisms which endanger any farm, be it the most remote & isolated. Such devastating diseases as Newcastle disease, Fowl Pox, Infectious Laryngotracheitis, Infectious Bronchitis in poultry & Infectious Bovine Rhinotraceitis, Bovine Viral Diarrhea, clostridial infections in cattle & sheet are only a few.
Since Louis Pasteu's vaccines against Fowl Cholera and Rabies, numerous vaccines have been developed and successfully applied under various conditions all over the globe. These different types can be divided into two major categories: Live Vaccines & Inactivated or Killed Vaccines. Live Vaccines utilize living cultures of attenuated or naturally mild strains of the microorganism, while inactivated vaccines use the natural or another form of the disease-causing agent in a biologically inactive state.
There are several basic differences between these two categories of immunizing preparations. Live Vaccines have the advantages of:
Mass application to large populations under some circumstances (via drinking water, aerosol spray, etc.) PA0 A relatively small quantity of the immunizing agent is necessary to illicit immunity, because the live organism multiplies in the body of the vaccinated animals. PA0 Live Vaccines are usually more economical and cheaper to apply. PA0 They require relatively small space under cold storage. PA0 The immunity which results from vaccination of animal with live vaccines is often very short lived, necessitating repeated additional vaccinations. PA0 The safety of using an inactivated (killed) microorganism is obvious, and constitutes the major advantage over live vaccines. PA0 Inactivated vaccines are more expensive to produce because they have to contain a larger mass of the antigen in order to ellicit a good immune response; PA0 Some inactivation methods alter the surface antigen configuration and thus the specificity of the vaccine is reduced. PA0 Inactivated vaccines must be applied by individual injection of every animal, and cannot be applied by mass application methods. PA0 They occupy sizeable volumes and weights which require cold storage. PA0 They often cause reactions at the site of injection; in the case of oil adjuvants, these may become granulomatous. PA0 A live vaccine is used, which contains a relatively small quantity of antigen, which is a cost saving factor, as live vaccines are much cheaper than inactivated ones. The live vaccine multiplies in the vaccinated animal's body. PA0 The live vaccine is packaged in very small vials which require small cold storage space, while the w/o emulsion component is very stable and does not require cold storage. It has a long shelf life and can be kept under any reasonable storage conditions. PA0 The resultant immunity generally surpasses the results obtained from a single live vaccine application. PA0 It is unnecessary to combine two vaccination applications; PA0 It actually combines the "priming" and the "booster" vaccinations into one step, so as to make the whole process time and labor saving.
On the other hand, live vaccines have an important disadvantage:
Inactivated vaccines are different from live ones. Their advantages are:
Inactivated vaccines are often coupled with an Adjuvant, a chemical or biological entity which serves to enhance the immunizing capability of the inactivated microorganism. As combined antigen and adjuvant the inactivated vaccine becomes potent and effective. Such vaccines have a longer lasting effect than live vaccines.
Their disadvantages are also of importance:
Adjuvants, as mentioned above, have become important components of inactivated vaccines. There exists biological adjuvants, such as bacterial endotoxins; pneumococcus capsular extracts of Lipopoly-saccharides (LPS) nature; whole Mycobacterium organisms and subunit components of mycobacterial cell walls. Irritants such as saponin have served as adjuvants in various vaccines. Surface-active chemical entities have been tried, such as detergents, polyols, polyanionics; and also polynucleotides have been used. Adjuvants have been used which produce physical configurations to "present" the antigen in a protected form and slow release mechanism such as Liposomes; or in a different concept--the antigen is adsorbed onto molecules of Aluminum hydroxide gel.
One of the most widely used adjuvants is the water-in-oil (w/o) emulsion, of the Freund's Incomplete Adjuvant (F.I.A.) type. This system includes and aqueous (or water) phase in which the antigen is dissolved or suspended; and oil phase in which the water phase is suspended as small droplets; and interface between the water & oil phases, usually lined by emulsifiers which stabilize the emulsion.
W/O emulsified vaccines are very potent and effective immunizing agents. The exact mechanism of immune potentiation is not very well proven. It is believed to be either an antigen-presentation mechanism; a depot of antigen which remains for extended periods of time at the injection site; a substance which attracts large numbers of macrophages which then enhance the immune response, or all these whichever is the real mechanism the net effect is that w/o emulsified vaccines are very good immunogens. Animals so vaccinated usually respond with a high level of immunity-both humoral & cellular; the immunity is very long lasting, often extending over a year or two; the levels of immunity obtained in animal populations are usually very uniform.
Under some circumstances, and in order to reduce the reaction at injection site a double emulsion is used; the water-in-oil-in-water (w/o/w) emulsion. This type of vaccine is very similar to the w/o type.
Best results of vaccination with w/o type vaccines are obtained in poultry for example, when a live & inactivated vaccine are combined. This can be done simultaneously, on one day old chicks, turkey poults, etc. vaccinated against Newcastle disease at the hatchery. Chicks are vaccinated by spray, by aerosol, or by occular or nasal drop methods with a live Newcastle disease vaccine such as the Hitchner B1 strain & at the same time they are injected with a small volume (usually 0.1-0.2 ml) of inactivated w/o emulsified Newcastle disease vaccine.
This combined vaccination results in an immunization which overcomes the maternal (passive) immunity of the chicks, which ordinarily interferes with active immunization of young chicks. (D. Warden, I. G. S. Furminger & W. W. Robertson, 1975; W. W. Robertson, et al 1976).
Later on it was realized that "priming" an animal with a live vaccine and "boosting" the immunity with an inactivated w/o vaccine later results in excellent & lasting immunity. It has become standard procedure with poultry breeders to vaccinate growing pullets once or more with live vaccines and then follow up with w/o emulsified killed vaccine before point-of-lay. Thus breeding & laying hens are immunized to obtain a high and uniform level of immunity which lasts throughout the laying period (some 10 to 12 months) or longer. This is practiced in the cases of Newcastle Disease, Infectious Bursal Disease ("Gumboro Disease"), Infectious Bronchitis, Viral Arthritis, etc. In all these cases, the live vaccines are composed of attenuated or naturally occurring mild strains of the disease causing agents, which are harmless or very mild in the side reactions which occur following their use.